Heads-up instrument displays (HUD) for vehicles have been used to provide virtual images that appear to be located ahead of the vehicle windshield toward the front of the vehicle. These displays provide the advantage of increased safety since the operator does not have to divert attention significantly from viewing the outside to check instruments.
Most conventional windshields are made by placing a layer of adhesive, e.g. polyvinyl butyral (PVB) or polyurethane, which has a substantially uniform thickness between two sheets of glass and laminating the sandwiched assembly in an autoclave. In a conventional HUD, where the image source is located away from the windshield, the light from the image source reflects off the windshield toward the viewer who sees the image floating in space beyond the windshield. If a conventional windshield is used, the viewer sees two separated images, one from the inside surface of the windshield and one from the outside surface. When these two images do not align in the viewer's sight, a ghost image is seen which interferes with acceptable viewing.
Approaches to reduce the ghost image problem include use of an adhesive interlayer disposed within the windshield with the interlayer having a predetermined wedge angle. The wedge angle is selected pursuant to the anticipated installation angle of the windshield, thickness of the glass sheets, and angle of incidence of the projected image so as to substantially superimpose the images generated from a dashboard display device that is reflected from the inside surface and outside surface of the windshield into a substantially singular image in the eyes of the vehicle operator. Among the approaches are those disclosed in U.S. Pat. No. 5,013,134 and Published PCT Application WO 91/06031 and U.S. Pat. No. 5,639,538.
The wedge angle for HUD projection devices is established by using an adhesive sheet having a wedge-shaped thickness profile extending continuously across the width from edge to edge of the sheet. Such a sheet and a process for preparing the sheet are disclosed in U.S. Pat. No. 5,087,502.
Web handling problems are encountered in manufacturing and transporting webs having a wedge-shaped thickness profile extending continuously across the width of the sheet. Web walking is the term used to describe sheeting that moves uncontrollably in the transverse direction as it is conveyed or processed. Telescoping is a term that describes what occurs when the core or individual layers of a rolled web becomes offset, or unevenly rolled. Web walking and telescoping damage the web and render it difficult or impossible to use in preparing windshields. These problems also limit tinting of the web. While special web handling equipment and procedures can be used with varying degrees of success, this is expensive and usually requires duplicating existing equipment used in preparing conventional adhesive sheeting. Making only short rolls may alleviate these problems. However, short rolls necessitate frequent changing of rolls during production and by the customer who processes the sheeting into windshields. In addition, shapes provided by prior art process often require additional trimming of the web, as extruded, resulting in an economic loss due to scrap created by trimming. Also, some of the prior art process requires a wider extrusion line due to the additional trimming.
It would therefore be advantageous to provide adhesive sheeting suitable for HUD windshield manufacture that does not require special web handling equipment and procedures or the creating of scrap that must be disposed of or recycled. Changing the shape of the web as extruded permits production on a more narrow extrusion line thus assuring supply to the market and increasing the flexibility of existing manufacturing facilities.
Shapes which minimize overall thickness while maintaining overall functional utility provide further economic advantage in material savings. Asymmetrical shapes provide capability to produce wider widths on narrow lines. Asymmetrical shapes can also be utilized to produce multiple wedge shapes simultaneously thus providing additional economic advantage by minimizing the frequency of long transition times to multiple shapes required for specific applications.